How Much Does It Cost To Go 80 vs 75?

Two years ago Governor Dennis Daugaard signed a bill into law that raised the maximum interstate highway speed limit in South Dakota to 80 miles per hour. What a lot of people didn't realize was that this same bill also raised the tax on gas by one percent.

The reaction by most people I know to the higher speed limit was positive. A lot of people said they usually went 80 on the interstate so now they could go a little faster. But, in most cases, going five miles per hour faster costs more. But how much more does it cost?

Without using a bunch of really complicated science I conducted a simple experiment last weekend. We drove our family Suburban to Rapid City for a friend's wedding. We had half a tank of 10 percent ethanol gas in it when we left. On the way out I drove the max speed limit of 80 miles per hour. We filled up in Oacoma and topped off the tank with the same fuel. When we filled up in Rapid the next day, with the same fuel, I figured out the mileage.

11.9 miles per gallon.

On the way back to Sioux Falls I set the cruise at 75 miles per hour for the entire trip. Of course there is more downhill than uphill driving coming east than going west so that was a factor, but that is mainly in the last 100 miles. My wife filled up with gas after we got home and unpacked.

15.9 miles per gallon.

Going 80 miles per hour in our Suburban, which holds 31 gallons of fuel, it would take 29.5 gallons of fuel to travel those 351 miles. At 75 it only takes 22.1 gallons fuel.

At the faster speed in our vehicle we would be running on fumes. Going slower we would still have nearly 9 gallons of fuel and be able to drive approximately another 140 miles before going dry.

Going faster saves you time so it may be worth it. If you can drive the 351 miles without stopping at 80 miles per hour it would take 4 hours 23 minutes. At 75 miles per hour it would take 4 hours and 41 minutes. If gas is $2.59 per gallon, as it was today when I came to work, it would cost about $21 more to save 18 minutes of drive time.

Of course, these calculations are done based on our gas guzzling Suburban. Every vehicle will be completely different. In my truck, which can only get 15+ miles per gallon going 70 or less, it really gets bad mileage. On my way to Pierre for a deer hunting trip I was in a hurry going 80, with two passengers and a ton of gear, I got 9 miles per gallon.

Is this as big of a deal when driving a car that gets much better mileage? Obviously not. It would likely cost comparatively less to go faster than our car. But it will still cost more to go faster. That's where the legislature is raising more money in taxes. It's a small amount for one person to pay per tank of fuel. But on the way out we paid an extra $1.68 in gas tax that we also didn't pay on the way home.

I don't mind the tax, or the incentive the state gave us to pay more in taxes. But overall I like spending less. So you will probably pass me going down the interstate.